Mercury switch



July 24, 1934. c. H. LARsoN MERCURYl SWITCH Filed sept. 1, 1955 5 sheets-sheet l1 Il lm! Illu lvllxllll; l l

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S Nll Q wv C. H. LARSON MERCURY SWITCH July 24, 1934.

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Patented July 24, v.1934

UNITED i srATEs MERCURY SWITCH Carl H. Larson, Elkhart, Ind., assignor to The Adlake Company, a corporation of Illinois Application September 1, 1933, Serial No. 687,742

29 Claims.

This invention relates to a time delay relay in which a mercury switch serves to make or break an electrical circuit at given intervals of time after the functioning of the switch actuating mechanism.

The time delay relays which have heretofore been used have either been large and cumbersome mechanical relays using clock mechanism where long time delays are required or tilting mercury switch relays in which an obstruction is used to impede the longitudinal ow of mercury toward or away from closed circuit position. The mechanical relays, in addition to being massive, are very expensive, while the tilting mercury switch relays are inaccurate and severely limited in the time delay which they are capable of effecting.

The principal objects of this invention, therefore, are to provide a relay which is far simpler in construction, much less expensive to manu- 20 facture and more accurate and reliable in operation than either of the two types of relays above mentioned; which has a wide range of time delays that it is capable of effecting which can produce substantially equal time delays when closing or opening a circuit, or unequal time delays, according to requirements; and`which is also capable of delaying the closing of a circuit for a given period of time after it has been acted upon by a control circuit but immediately opens the circuit when the original condition ofthe control circuit is restored.

Further and other objects and advantages of 1 this invention will become apparent, as the disclosureproceeds, and the description is read in conjunction with the accompanying drawings, in which Fig. 1 is a sectional view illustrating the open circuit position of a slow make quick break front contact relay;

Fig. 2 shows the action which takes place immediately after energizing the relay coil;

Fig. 3 shows the closed circuit position of'the switch;

Fig. 4 illustrates the momentary carrying up of the mercury within the switch plunger when the relay coil is de-energized; f

Fig. 5 is a perspective view of the plunger;

Figs'. 6 and '7 illustrate how the switch of Figs. 1 to 4, inclusive, may be converted into a slow 50 make retarded break'switch by merely lowering the position of the air gip;

Fig. 8 shows a slow make slow break switch immediately after the plunger has been pulled down by the coil;

Fig. 9 illustrates how a time delay is effected (Cl. 20D- 97) in breaking the circuit using the ,switch of Fig. 8;

Fig. 10 is a perspective view of the plunger. lshown in Figs. 8 and 9; h

Figs. 11, 12 and 13 are views illustrating a slow make quick break contact relay, Figs. 11 and 13 60 showing the closed circuit-and open circuit positions, respectively, of the switch, and Fig. 12 showing the momentary carrying up of mercury within the plunger when the associated relay coil is energized.

Fig. 14 shows the switch of Figs. 11 to 13, inclusive, adjusted for effecting a slow breakof the circuit, as well as a slow make;

Fig. 15 illustrates a slight modication. of the switch of Fig. 14, the slot at the bottom of the plunger being omitted; and

Fig. 16 illustrates how a circuit may be immediately broken upon energization of the coil by lifting the plunger completely out of the mercury.

At the outset, 'it should be understood that any and all specic description found in this speciiication is for the purpose of disclosure and should not be interpreted as imposing-limitations upon the appended claims, unless required by the prior art.

Terminology.

Throughout this specification the followirn.r terms are to be given the indicated connotations:

Front contact relay. A relay in which the electrical circuit is open when the associated relay coil is de-energized.

Back contact relay. A relay in which the electrical circuit is closed when the associated relay coil is de-energized.

Slow make relay. A relay in which thereI is a time delay between the functioning of the switch actuating mechanism and the closing of the electrical circuit controlled by the relay.

Slow break relay. A relay in which there is a time delay between the functioningv of the switch actuating mechanism and the opening of the electrical circuit controlled by the relay.

Retarded break relay. A relay in which there is a time interval between the functioning of the switch actuating mechanism and the openingof the circuit, but in which the actual breaking oi' the circuit is accomplished by a quick separation of the conductors at the moment when the circuit is opened.

Front contact relay, slow make, quick break- (Figs. 1 to 5) In Fig. 1 the reference character 10 designates a relayl coil vwhich includes a spool 11 of insull and 13 is a switch 16 comprising a glass envelope 17 through the bottom of which electrodes 18 and 19 are sealed. The central electrode 18 is'surrounded by an insulating sleeve 20 which extends to a point 21 just below the top of the electrode. The envelope is lled to a given level with a quantity of mercury 22 and the condition of the electrical circuit between the electrodes 18 and 19 is determined by the level of the mercury.

The displacer or plunger 23 of magnetic material normally fioats upon the mercury, as shown Vin Fig. l, but is adapted to be drawn downwardly through magnetic influence to raise the mercury level and electrically connect the electrodes 18 and 19. The top of the displacer is interiorly tted with a metal sleeve 24 which carries a porous thimble 25 at its upper end. Preferably, the thimble is made of a ceramic material, known as alundum, which is manufactured by the Norr ton Company of Worcester, Massachusetts. One of the three principal grades, R-A-84 dense, R-A-360 medium and RA98 coarse, may be selected according to the specification of the switch.

Obviously, the metal sleeve 24 may be omitted, if desired, and the ceramic thimble 25 attached directly to the plunger or displacer 23.

A slot 26 is cut in the upper Vside wall of the plunger and extends to a point slightly below the bottom edge of the sleeve 24, Any kind of an opening immediately below the bottom of the sleeve would do as well.

In order to increase the buoyancyl of` the plunger, it is desirable to equip the bottom of the plunger with a ceramic insert 27 which may be of glass, alundum or other suitable material.

Coil springs 28 and 29 made of non-magnetic` material are provided to protect the switch envelope from damage during shipment. Preferably, the plunger 23 is made with sulcient buoyancy -so when the coil l0 is de -energized, the spring 28 will be slightly compressed by the plunger.

After the displacer with its associated parts and the mercury have been placed in the switch envelope, the switch is evacuated and a suitable gas ll is inserted, such, for example, as helium, hydrogen or helium hydride. Thereafter, the top of the switch envelope is sealed oi, in accordance with the usual practice.

'I'he switch is shown in Fig. 1 in its normal deenergized state with the circuit between the electrodes 18 and 19 broken.

As soon as the relay coil is energized, the switch plunger 23 is lowered to the position lshown in Fig. 2 becausethe air gap 14 is traversed by a strong magnetic field and the plunger moves to a position which reduces the air gap to a minimum.

As the plunger starts on its downward movement, the mercury level on boththe inside and outside of the plunger begins to rise due .tothe displacement of mercury by the plunger. As soon as the mercury level within the plunger reaches the bottom of the sleeve 24 the gas entrapped within the thimble forces the mercury away from the bared portion of the electrode 18, while the mercury level on the outside is being raised due to further displacement of the mercury by the plunger. 'I'his is clearly shown in Fig. 2.

The porous material of which the thimble 25 is composed allows the entrapped gas to slowly escape from the plunger, and as this happens the mercury levels on the inside and outside of the plunger tend to equalize. The time which is required for the equalization to take place depends upon the porosity of the ceramic material, but eventually the mercury assumesa common level on both the inside and outside of the plung' er,'as shown in Fig.- 3. Obviously, as soon as the mercury level within the plunger rises above the top of the insulating sleeve 20, the electrical circuit through the electrodes 18 and 19 is corn- When the coil 10 is de-energized, the buoyancy of the plunger causes it to rise to the position shown in Fig. 4. Momentarily, mercury is carried upwardly within the sleeve 24 due to the inability ofthe gas to enter the plunger through the ceramic thimble fast enough to maintain equal pressures on the inside and outside of the thimble. However, as soon as the mercury level on the outside drops below the bottom of the slot 26, gas can enter the plunger through the small openingbeneath thesleeve 24, which results in the mercury column within the plunger being undermined, and the mercury levels on both the inside and outside of the plunger quickly fall to the common level at which they are shown in Fig. 1.

It will, therefore, be seen that the switch shown in Figs. 1 to 5 inclusive effects a. slow making of @che electrical circuit between the electrodes 18 and 19 when the coil 10 is energized, butthereafter, on de-energizing the coil, the plunger rises immediately to its normal position, shown in Fig. 1, thus promptly breaking the electrical circuit, although, momentarily, mercury is carried up within the plunger, as shown in Fig. 4.

Front contact relay, slow make, retarded break- (Figs. 6 and 7) By merely lowering the position of the air gap 111, the switch shown in Figs. 1 to 5, inclusive, may

be made to operate as a slow make retarded breakswitch. This may be explained as follows: When the plunger is lowered by the coil 10, it is pulled deeper (Fig. 6) into the mercury than at first, due to the lowered position of the air gap. Consequently, when the mercury levels oi after suflicient time has elapsed for the necessary volume of gas to pass from the plunger through the thimble 25, the common level will be substantially above that in which it was shown in Fig. 3. This level is indicated by dotted line in Fig. 6.

. Thereafter, when the coil is de-energized, an amount of mercury will be carried up by the plunger, which, addedto the weight of the plunger itself, will not permit the buoyancy of the plunger to lift thebottom of the sleeve 24 above the outside mercury level, as indicated in Fig; 7, but as gas enters the plunger through the' thimble 25, the mercury level within the plunger recedes, thereby diminishing the weight of the plunger and causing the latter to slowly rise, the outside mercury level, however, remaining constant. As soon as suicient gas has entered through the thimble, the bottom of the sleeve 24 will be carried above the outside mercury level, whereupon the mercury column within the sleeve will be undermined and the circuit through the electrodes 18 and 19 quickly broken. e

The movement of the plunger when the coil is ole-energized is characterized 'by a preliminary 150 Frmzt contact relay, slow make, slow (Figs. 8 and 9) From the description of the slow make retarded break relay, it is apparent that if the slot 26 were omitted, the switch would function as a slow make, slow break relay. The same result, however, can be obtained by using a plunger of somewhat simpler construction. l

In Figs. 8 and 9, the switch 30 diifers'principally from the switch 16 in having a shorter central electrode 31 and further by being provided with a plunger 32 in which the ceramic thimble 33 is tted directly into the top of the plunger. The side walls of the plunger are imperforate so that gas can enter the plunger only through the thimble 33.

The position of the switch parts immediately after the coil has been energized is shown in Fig'. 8 and the dotted line 34 in the same ligure indicates the mercury level after the coil has been energized and sufficient time has elapsed to permit the inside and outside mercury levels to breakequalize.

Upon deenergizing the coil 10,- the plunger 32 rises to the position shown in Fig. 9 carrying a column of mercury upwardly within the plunger. 'I'his column slowly recedes as gas enters the plunger through the thimble 33 and at the same time the plunger slowly rises as its effective Weight is reduced. As soon as the mercury falls below the top of the insulation which surrounds the electrode 31, thev electrical circuit controlled by the relay is broken.

It is obvious that the relative time delays on making and breaking the circuit may be varied within limits by adjusting the position of the air gap, the height of the central electrode, the mercury fill, etc.

Back Contact, slow make, quick break-(Figs. 11 to 13, inclusive) In a back contact relay, it will be remembered the electrical circuit is closed when the relay coil is deenergized. Thus in Fig. 11 the plunger 40 has suiicient weight tolbring the mercury level to above the top of the insulating sleeve 4l which surrounds the central electrode 42, thereby causing the electrical circuit through the electrodes 42 and 43 to be closed. The pole sleeves 44 and 45 are so positioned that when a rragnetic eld passes between the two sleeves, the plunger 40 will be lifted to the position shown in Fig. l2 and will remain there as long as the coil is energized. In the switch shown in Figs. 11 to 13, a slot '46 is provided in the side wall of the plunger so that when the plunger is raised to the position shown in Fig. 12 the top of the slot wil be lifted above the outside mercury level, thus causing the column of mercury within the plunger to be undermined and fall away from the electrode 42. When this occurs, the mercury level, will besubstantially as shown in Fig. 13.

Obviously, when the relay afterward is deenergized, the plunger 40 will fall by gravity to a position somewhat above that shown in Fig. 11, forcing mercury away from the central electrode because of the entrapment of gas within the plunger. As the gas slowly escapes throughthe thimble 4'7, the mercury will rise to a suicient height to cover the central electrode and establish a circuit between the electrodes 42 and 43.

Back contact relay, slow make, slow break- (Figs.14 and 15) The switch lshown in Fig. 14 is identical with the one shown in Figs. 11 to 13, inclusive, but the air gap between the pole sleeves 44 and 45 issomewhat lower relative to the plunger so that the slot 46 of the plunger is not uncovered when the plunger is raised by energizing the relaycoil. In this way, the mercury actually carried up by the plunger recedes slowly as gas is admitted to the interior of the plunger and thereby effects a slow breaking of the electrical circuit.

Upon dropping the plunger by releasing the magnetic force, mercury is pushed away from the central electrode due to the entrapment of gas within the plunger, but the slow escape of gas through the thimble 47 permits the inside and outside levels to equalize, and finally establish the circuit.

Since the slot 46`serves no useful purpose when the air gap is arranged, as shown in Fig. 14, it may be eliminated, as shown in Fig. 15, giving a wider range for adjustment of the air gap and a consequent lengthening of the'time delays.

Back contact relay, slow make, quick break- (modified form Fig. 16)

plunger wall and is adapted to have a coil spring 53 fitted over its projecting end. A similar spring 54 is fitted over the ceramic thimble 55 which closes the opening 56 at the top of the plunger.

The ceramic sleeve 52 materially increases the buoyancy of the plunger and makes it unnecessary to lift the plunger but a slight distance in order to break the circuit. When the coil is deenergized, the plunger drops into the mercury and slowly permits the mercury around the central electrode 57 to rise and connect the bared portion of the electrode with the conductor 58.

.in all forms of the relay, the plungers are preferably provided with lugs 59, as best shown in the perspective views of Figs. 5 and 10. The lugs serve to guide the plunger in its vertical travel within the switch envelope.

instead of using a thimble of porous material at the top of the plungers to obtain a slow gas leakage, a small opening could be provided having the appropriate diameter for a given time delay. If a relatively long time delay is required, the opening may be restricted by inserting a tapered plug, which permits the size of the opening to` place with dental cement. Fine adjustment of time delay maybe obtainedjby daubing the exposed part of the thimble with cement until time delay is brought down to specification.

What I claim, therefore, is:

l. In a mercury switch` adapted to eiect a slow make and quick break of an electrical circuit when associated with a relay coil, the combination of a vertical switch envelope, spaced electrodes in the envelope, a charge of mercury adapted to open or close an electrical circuit through the electrodes according to its level, a plunger movable within the envelope in response to the coil for changing the mercury level, said plunger being telescoped over one of the electrodes and closed at the top except-for one or more restricted openings so that when the plunger is lowered, mercury is held away from said electrode for a` given time interval, said plunger also having an .opening positioned so that when the plunger is in its raised position, the mercury coluimn carried upwardly within the plunger is undermined and immediately falls away from thel said electrode.

2. In a mercury switch adapted to effect a slow make and quick break of an electrical circuit when associated with a relay coil, the combination of a vertical switch envelope, spaced electrodes in the envelope,'a mercury fill adapted to make or break an electrical circuit through the electrodes according to its level, a plunger movable within the envelope in response to the coil for changing the mercury level, and means adapted tol retard the movement of the mercury to one of said electrodes, and means for undermining the mercury column around said electrode as soon as the plunger is moved to its raised position.

3. In a mercury switch adapted to eiect a slow make and quick break of an electrical circuit when associated with a relay coil, the combination of a vertical switch envelope, spaced electrodes in the envelope, a mercury fill adapted to make or break an electrical circuit through the electrodes accordingto its level, a plunger movable within the envelope in response to the coil for changing the mercury level, and means carried by the plunger adapted to retard the movement of the mercury to one of said electrodes, and means for undermining the mercury column around said electrode as soon as the plunger is moved to its raised position.

4. In a mercury switch adapted to effect a slow make and quick break of an electrical circuit when associated with a relay coil, the com. bination of a vertical switch envelope, spaced electrodes in the envelope, a charge of mercury adapted tomake or break an electrical circuit through the electrodes according to its level, a plunger movable within the envelope in response to the coil for changing the mercury level, said plunger -normally iloating on the'mercury and displacing an insuillcient quantity of mercury to bridge the electrodes when the relay coil is deenergized, means adapted to retard the movement of mercury to one of said electrodes, andvmeans for undermining the mercury column around said electrode as soon as the plunger is allowed to rise to its oating position by de-energization of the coil thereby causing a quick breaking of the circuit.

5. In a mercury switch adapted to effect a slow make and quick break of an electrical circuit when associated with a relay coil, the combination ofia vertical switch envelope, spaced electrodes in the envelope, a charge of mercury adapted to make or break an electrical circuit through theelectrodes according to its level, a plunger movable within the envelope in response to the coil for changing the mercury level, said` plunger normally displacing a suiiicient quantity of mercury to bridge said electrodes when the relay coil is de-energized, means adapted to retard the movement of mercury to one .of said electrodes,` and means for undermining the merelectrodes in the envelope, a charge of mercury,

adapted to make or break an electrical circuit through the electrodes' according to its level, a plunger movable within the envelope in response to the coil for changing the mercury level, said plunger being telescoped over one of the electrodes and having oneor more restricted openings adjacent to its top whereby the movement of mercury to and away from the said electrode is retarded by the unequal gas pressures on the in' sideand outside of the plunger, said plunger also having an opening positioned so that when the plunger is in its raised position, the mercury column carried upwardly within the plunger is immediately undermined and falls away fromthe said electrode.

7. In a mercury .switch adapted to effect a slow make andquick break of an electrical circuit when associated with a relay coil, the combination of a vertical switch envelope, spaced electrodes in the envelope, a charge of mercury adapted to make or break an electrical circuit through the Yelectrodes according to its level, a plunger movable within the envelope in response to the coil for changing the mercury level, means for retarding the movement of `mercury to and away from one of said electrodes when the mercury level is changed by movement of the plunger, said plunger having an opening in one of its side walls positioned so that when the plunger is in its raised position, the mercury column carried upwardly within the plungeris undermined and immediately falls away from said electrode.

8. A plunger for use in a mercury switch of the class described, comprising a tubular body at least a portion of which is made of magnetic material, said plunger being closed at' the top except for one or more restricted openings, said plunger also having an opening piercing the side Wall of the plunger at a point slightly above the mercury level line of normal oating position.

9. A plunger for use in a mercury switch of the class described, comprising a tubular body at least a portion of which is made of magnetic material, a porous wall in the upper part of the plunger, said plunger being provided with an opening which pierces the side wall of the plunger at a point slightly above the mercury level line of normal floating position.

10. In a mercury switch adapted to eiect a slow make and a somewhat faster break of an electrical circuit when associated with a relay coil, a combination of 'a vertical switch envelope, spaced electrodes in the envelope, a charge of mercury adapted to open or close an electrical circuit through the electrodes according to its level,l a plunger movable within the envelope in response to the coil for changing the mercury level, said plunger being telescoped over one of the electrodes and closed at the top except for one or more restricted openings so that when the plunger is lowered upon energization of vthe coil, mercury is held away from said electrode for a given time interval, said plunger also having an opening positioned so that when the plunger is released by de-energization of the coil, it Will levitat'e by first a quick movement, then a retarded movement, and lastly by another quick' movement, which latter movement eiects a substantially instantaneous opening of the circuit.

11. In a mercury switch adapted to elect a slow make and slow break of an electrical circuit when associated with a relay coil, the combination of a vertical switch envelope, spaced electrodes in the envelope, a mercury Iill adapted to make or break an electrical circuit through the electrodes according to its level, a plunger movable within the envelope in response to the coil for changing the mercury level and means for retarding the movement of the mercury to and from one of said electrodes, said means comprising a po-rous wall adjacent to the top of the plunger.

l2. In a mercury switch, a sealedvertical envelope, spaced electrodes in the envelope, a gas fill, a charge of mercury adapted to close or open a circuit across the electrodes, according to its level, a, displacer movable within the envelope to displace the mercury, said displacer being hollow and telescopically movable over one of the electrodes, and a porous wall in the upper part of the plunger for delaying movement of the mercury col-umn within the displacer.

13. In a mercury switch, a switch envelope, spaced electrodes in the envelope, a gas fill, a quantity of mercury in the envelope adapted to be manipulated to make or break an electrical circuit through the electrodes, an inverted cupshaped member provided with a porous wall ad jacent to the top for retarding the movement of mercury withinfthe member, in at least one direction, by the gas pressure differential existing between the interior and exterior of the member, said electrodes being arranged so that the condition of the circuit is determined by the height of the mercury within the member with respect to a fixed point on the envelope.

14. In a mercury switch, a switch envelope, spaced electrodes in the envelope, a gas ll, a quantity of mercury in the envelope adapted to be manipulated to make or break an electrical circuit through the electrodes, an inverted cupshaped member telescoped over one of the electrodes and presenting a relatively large opening to the mercury and a limited opening to the gas,-

said latter opening consisting of a porous wall adapted to pass gas but slowly, means for changing the mercury level on the outside of the member whereby movement of the mercury within the member in at least one direction is retarded by the restricted flow of gas through the porous wall.

15. In a mercury switch, a switch envelope, spaced electrodes in the envelope, a gas iill, a mercury iill adapted to be manipulated to make or break an electrical circuit through the electrodes, a displacer normally displacing a suicient quantity of mercury to bridge the electrodes, but being adapted to be raised by magnetic force completely out of the mercury whereby the connection between the electrodes is broken, and means including a restricted gas passage for delaying the movement of mercury toward one of the electrodes when the ldisplacer is dropped from its raised position.

16. In a mercury switch, a stationary switch envelope, spaced electrodes in the envelope, a gas ll, a mercury ll adapted to be manipulated to make or break an electrical circuit through the electrodes, and'means for retardingtheni'ovement of the mercury to effect a given time delay in changing the condition of the electrical circuit, said means including a. porous wall, and means for creating unequal g-as pressures on opposite sides of the wall without moving the switch envelope.

17. In a mercury switch, a stationary switch envelope, spaced electrodes in the envelope, a-

gas fill, a mercury ll adapted to be manipulated to make or break an electrical circuit through the electrodes, and means for producing a time delay in changing the condition of the electrical circuit in the switch without moving the switch envelope, said means including a porous wall interposed between the gas ll and a column of mercury moving to and away from one ofthe electrodes, so that the retarded passage of gas through the wall will cause a corresponding retarded movement of the mercury colunm in at least one direction.

- 18. In a mercury switch, a; stationary switch envelope, spaced electrodes in the envelope, a gas fill, a mercury ll adapted to be manipulated to make or break an electrical circuit through the electrodes, and means for producing a time delay in changing the condition of the electrical circuit in the switch without moving the switch Y envelope, said means including a porous Wall retarding the passage of gas therethroughand in consequence retarding the flow of mercury to circuit opening or circuit closing position.

19. In a mercury switch, a stationary switch envelope, spaced electrodes in the envelope, avgas fill, a mercuryll adapted to be manipulated without movingv the switch envelope to make or break an electrical circuit through the electrodes, and means for retarding the movement of a portion of the mercury during such manipulation to 1 eliect a given time delay in changing the condition of the. electrical circuit, said means including a porous Wall through which gas is slowly passed by unequal pressures developed on opposite sides of the wall by the movement of the remainder of the mercury.

20. In a mercury switch, a sealed vertical envelope, spaced electrodes in the envelope, a gas fill, a charge of mercury adapted to close or open a circuit across' the electrodes, according' to its level, a displacer movable within the envelope to displace the mercury, and means for retarding movement of a portion of the mercury to effect a given time delay in changing the condition of the electrical circuit, said means including a porous wall arranged so that movement of the displacer creates unequal gas pressures on opposite sides of the wall.

21. In a mercury switch, a sealed vertical envelope, spacedelectrodes in the envelope, a gas ll, a charge of mercury adapted to close or open a circuit across the electrodes, according to its level, means for forming a gas trap around one of the electrodes to effect a given time delay, said means including a displacer for shifting the mera circuit across the electrodes, accor t to its level, a displacertelescopedover one of the electrodes for changing the mercury level, and `porous material closing the top of the displacer whereby a gas trap ot slow leakage is formed over the said electrode when the displacer is lowered.

23. In a mercury switch, a sealed vertical envelope, spaced electrodes in the envelope, a gas iill, a charge of mercury adapted to close or open a circuit across the electrodes, according to its level, a displacer movable within the envelope to displace the mercury, said displacer having a tubular body telescopically movable over one of said electrodes, and porous material closing the top of the displacer.

24. A displacer for use in a time delay mercury switch comprising a tubular body, at least a portion of which is of magnetic material, and a porous wall in the upper part of the displacer.

25. A plunger for use in a mercury switch of the class described comprising a tubular body at least a portion of which is of magnetic material, and a porous wall in the upper part of the plunger adapted to pass gas relatively slowly therethrough.

26. .A plunger for use in a switch of the class described comprising a sleeve of magnetic material having an inverted cup-shaped sleeve interiorly mounted in the rst named sleeve, said in ner sleeve including a porous wall.

27. A plunger for use n a switch of the class' described comprising an outer sleeve of magnetic material having a uniform external diameter slightly less than the internal diameter of the switch envelope, 'guide lugs on the sleeve for maintaining it at the appropriate distance from the envelope, and an inverted cup-shaped sleeve, interiorly mounted in the rst named sleeve, said inner sleeve including a porous wall.

28. In combination with a coil, a mercury switch comprising a. vertical switch envelope, spaced electrodes in the envelope, a mercury fill, a displacer including magnetic material acting in response to the coil for changing the mercury level, and means for retarding the movement of a. portion of the mercury to eiect a given time delay, said means including an inverted cup-shaped element providedv with a porous wall and ar- -ranged to form a pocket over said electrode.

29. A plunger for use in a mercury switch of the class described comprising a tubular body at least a portion of which is of magnetic material, a porous wall in the upper part of the plunger adapted to pass gas relatively slowly therethrough, and a dab of cement applied to the porous wall for restricting the rate of gas ow through the wall.

CARL H. LARSON. 

